1. Field of the Invention
The invention relates to a shift register circuit and a display apparatus using such a circuit and, more particularly, to an LCD (Liquid Crystal Display) or a TFT (Thin Film Transistor) active matrix display.
2. Description of the Related Art
Hitherto, in an amorphous silicon (a-Si) TFT-LCD, in order to reduce fabrication costs and driving IC costs, improve the reliability, and reduce an area of a non-display portion, a shift register built-in type system in which a shift register circuit for scanning gate lines is integrated simultaneously with TFTs of a pixel portion and mounted has been proposed. As a typical example of such a system, for example, JP-A-2004-348940 can be mentioned.
The conventional shift register circuit also disclosed in JP-A-2004-348940 has the following two problems: (1) stability of a gate terminal of an output transistor for driving the gate line; and (2) a through-current.
First, the problem (1) will be explained. In the conventional a-Si TFT, there is a problem of “threshold shift” in which if a voltage which is higher (lower) than that at each of a first terminal and a second terminal is continuously applied to a gate terminal (such a state is called a “DC stress”), a threshold voltage of the TFT is shifted to a higher (lower) voltage. It has been known that a similar problem occurs not only in the a-Si TFT but also in an organic TFT or the like.
In the case of driving the output transistor by using such a TFT, if a time during which the high-level voltage is applied to the gate terminal of the TFT becomes long, the threshold shift progresses and driving ability deteriorates in accordance with the elapse of time. If the driving ability of the TFT deteriorates, upon preparation for the operation of the self stage, a capacitor connected to the gate terminal of the output transistor cannot be charged to an sufficient voltage and a defective operation is caused.
The problem (2) will now be explained. In the conventional shift register circuit, upon preparation for the operation of the self stage, the through-current flows through two transistors in the ON state. Since the through-current is so-called a wasteful current which does not directly contribute to the driving of the gate line, it is desirable that such a current does not exist.